# US 10,374,786 B1 · Methods of estimating frequency skew in networks using timestamped packets > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 10,374,786 B1 - Document ID: US10374786B1 - Status: Granted - Kind: B1 - Application number: 14/992,486 - Filed: 2016-01-11 - Granted: 2019-08-06 - Earliest claimed: 2012-10-05 - Assignees: Integrated Device Technology, Inc. - Applicants: Integrated Device Technology, Inc. - Inventors: Frederic Mustiere; Russell Smiley; Felix Duong; Alain Trottier - Web page: https://patentgrep.com/patent/US10374786B1 ## CPC - H04L 7/0016: {correction of synchronization errors} - H04J 3/0635: {Clock or time synchronisation in a network (timer in protocols H04L69/28)} - H04J 3/0661: {using timestamps} - H04J 3/0667: {Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays (arrangements for monitoring round trip delays in packet switching networks H04L43/0864)} - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) - H04J: MULTIPLEX COMMUNICATION (transmission in general H04B; peculiar to transmission of digital information H04L5/00; systems for the simultaneous or sequential transmission of more than one television signal H04N7/08; in exchanges H04Q11/00; stereophonic systems H04S) ## Abstract Methods of estimating frequency skew in a packet network include determining a representation of a packet delay variation (PDV) sequence from an initial estimate of frequency skew between master and slave devices in the packet network and timestamps transmitted therebetween. An operation is performed to extract a statistical mode from an empirical probability distribution function (PDF) of the representation of the PDV sequence, where the statistical mode corresponds to a value at which the PDF has its maximum value. The, an updated estimate of the frequency skew is generated by determining a slope between timestamps at indices associated with a plurality of points in the representation of the PDV sequence that are within a range of the statistical mode. ## Claims ### Claim 1 A method of estimating and adjusting frequency skew of clocks in a packet processing network, comprising: transmitting packets containing timestamps between master and slave devices in the packet processing network, which are synchronized with respective clocks and separated from each other by a packet network over which: (i) a first plurality of the packets are transmitted in-sync with a master clock generated within the master device, and (ii) a second plurality of packets are transmitted in-sync with a slave clock generated within the slave device; generating an initial estimate of frequency skew of the slave clock relative to the master clock, from the timestamps within the first and second pluralities of the packets; determining a representation of a packet delay variation (PDV) sequence from the initial estimate of the frequency skew and the timestamps; determining a statistical characterization of the representation of the PDV sequence; extracting a statistical mode from the statistical characterization and determining first indices of a first plurality of points in the representation of the PDV sequence that are above the statistical mode and second indices of a second plurality of points in the representation of the PDV sequence that are below the statistical mode, said extracting comprising extracting the statistical mode from an empirical probability density function (PDF) of the representation of the PDV sequence, where the statistical mode corresponds to a value at which the PDF has its maximum value; generating an updated estimate of the frequency skew by determining a slope between the timestamps at the determined indices, said slope specifying a change in master and slave time-of-day (TOD) offsets associated with the timestamps relative to a change in index of the timestamps; and adjusting, based on the updated estimate of the frequency skew, at least one of the master and slave clocks in the master and slave devices, respectively, to thereby improve synchronization therebetween. ### Claim 2 (depends on 1) The method of claim 1, wherein said generating an initial estimate of frequency skew comprises normalizing the timestamps. ### Claim 3 (depends on 2) The method of claim 2, wherein said determining a representation of a packet delay variation (PDV) sequence comprises determining a representation of a packet delay variation (PDV) sequence from the initial estimate of the frequency skew and the normalized timestamps; and wherein said generating an updated estimate of the frequency skew comprises generating an updated estimate of the frequency skew by determining a slope between the normalized timestamps at the determined first and second indices. ### Claim 4 (depends on 1) The method of claim 1, wherein the packet network is electrically coupled to the master and slave devices. ### Claim 5 (depends on 3) The method of claim 3, wherein said extracting a statistical mode and said generating an updated estimate of the frequency skew are performed iteratively to thereby narrow a region around the extracted statistical mode. ### Claim 6 (depends on 1) The method of claim 1, wherein said adjusting comprises adjusting a frequency of the slave clock in the packet processing network to thereby improve a frequency skew of the slave clock relative to the master clock. ### Claim 7 (depends on 6) The method of claim 6, wherein packet processing network operates in a non-stationary network environment. ### Claim 8 (depends on 1) The method of claim 1, wherein said transmitting, generating, determining, extracting and adjusting steps are performed by the packet processing network. ### Claim 9 A method of estimating and adjusting frequency skew of clocks in a packet processing network, comprising: determining a representation of a packet delay variation (PDV) sequence from an initial estimate of frequency skew between master and slave devices in the packet processing network and packets containing timestamps transmitted therebetween, said packets comprising; (i) a first plurality of packets transmitted across a packet network in-sync with a master clock within the master device, and (ii) a second plurality of packets transmitted across the packet network in-sync with a slave clock within the slave device; determining a statistical characterization of the representation of the PDV sequence from the initial estimate of the frequency skew and the timestamps; extracting a statistical mode from the statistical characterization by extracting the statistical mode from an empirical probability distribution function (PDF) of the representation of the PDV sequence, where the statistical mode corresponds to a value at which the PDF has its maximum value; generating an updated estimate of the frequency skew by determining a slope between timestamps at indices associated with a plurality of points in the representation of the PDV sequence that are within a range of the statistical mode, said slope specifying a change in master and slave time-of-day (TOD) offsets associated with the timestamps relative to a change in index of the timestamps; and adjusting, based on the updated estimate of the frequency skew, at least one of the master and slave clocks in the master and slave devices, respectively, to thereby improve synchronization therebetween. ### Claim 10 (depends on 9) The method of claim 9, wherein the packet network is electrically coupled to the master and slave devices. ### Claim 11 (depends on 9) The method of claim 9, wherein said adjusting comprises adjusting a frequency the slave clock in the packet processing network to thereby improve a frequency skew of the slave clock relative to the master clock. ### Claim 12 (depends on 11) The method of claim 11, wherein packet processing network operates in a non-stationary network environment. ### Claim 13 (depends on 9) The method of claim 9, wherein said determining, extracting, generating and adjusting steps are performed by the packet processing network. ### Claim 14 A method of estimating and adjusting frequency skew of clocks in a packet processing network, comprising: generating an initial estimate of frequency skew from timestamps transmitted in packets between master and slave devices in the packet processing network, which are separated from each other by a packet network, said packets comprising; (i) a first plurality of packets transmitted across the packet network in-sync with a master clock within the master device, and (ii) a second plurality of packets transmitted across the packet network in-sync with a slave clock within the slave device; determining a representation of a packet delay variation (PDV) sequence from the initial estimate of the frequency skew and the timestamps; determining a statistical characterization of the representation of the PDV sequence from the initial estimate of the frequency skew and the timestamps; extracting a statistical mode from the statistical characterization, said extracting comprising extracting the statistical mode from an empirical probability distribution function (PDF) of the representation of the PDV sequence; generating an updated estimate of the frequency skew from a set of modal points extracted from the probability density function (PDF) of the representation of the PDV sequence, said modal points aligned to a line having a slope that specifies a change in master and slave time-of-day (TOD) offsets associated with the timestamps relative to a change in index of the timestamps; and adjusting, based on the updated estimate of the frequency skew, at least one of the master and slave clocks in the master and slave devices, respectively, to thereby improve synchronization therebetween. ### Claim 15 (depends on 14) The method of claim 14, wherein said generating an initial estimate of frequency skew comprises normalizing the timestamps. ### Claim 16 (depends on 14) The method of claim 14, wherein the packet network is electrically coupled to the master and slave devices. ### Claim 17 (depends on 14) The method of claim 14, wherein said adjusting comprises adjusting a frequency of the slave clock in the packet processing network to thereby improve a frequency skew of the slave clock relative to the master clock. ### Claim 18 (depends on 17) The method of claim 17, wherein packet processing network operates in a non-stationary network environment. ### Claim 19 (depends on 14) The method of claim 14, wherein said generating, determining and adjusting steps are performed by the packet processing network.